This invention relates to a high-efficiency heat exchanger/heater for use in boilers, vapor generators, spa or pool heaters, engine exhaust heat recovery units, and other heat exchangers/heaters employing a relatively short, small-diameter tube.
Previous inventions have employed internal heating elements secured to the interior of the firetube to promote the transfer of heat from hot gasses flowing within the firetube through the firetube walls and into the medium to be heated. U.S. Pat. No. 5,913,289, for example, teaches a firetube heat exchanger utilizing fins formed of longitudinal corrugations. These axially aligned fins cover the inside wall of the firetube and substantially increase the internal surface area over that of the bare tube. The fins are formed out of corrugated sheet-metal brazed to the wall of the tube. To prevent the fins from overheating, the leading edge of the corrugations that would otherwise permit hot gasses to enter and flow outside the corrugations along the tube is blocked off by a ring flange brazed to the tube wall at the start of the corrugations. Hot gasses enter the space outside the corrugations adjacent to the tube through slots cut into the bases of the fins near the tube wall. The slots are sized to allow approximately half of the hot gas to pass though the slots and flow outside the corrugations, with the remainder forced to flow inside the corrugations.
U.S. Pat. No. 5,913,289 further discloses a core plug which fills the space inside the inner radius of the corrugations. The core plug forces the gas to flow near the fins, which results in a higher heat transfer coefficient. The core plug is also tapered over a length of several inches to gradually force the gas into the space between the corrugations.
While the previous invention reduces the temperatures of the fin tips, the construction of the fins results in significant thermal stress. In a fired heater, the average fin temperature is always hotter than the tube wall to which it is attached. Therefore, if the thermal expansion coefficients of the tube and fin are similar, the fin expands more than the tube. This puts the longitudinal fin in a state of compressive stress. If the stress is high enough, the longitudinal stress may cause the fin to buckle and even cause the fin-tube bond to fracture.
As an order of magnitude estimate of the compressive stress, in a case where the average fin temperature is 500xc2x0 F. hotter than the tube, the thermal expansion coefficient of both the tube and the fin is 6xc3x9710xe2x88x926xc2x0 F.xe2x88x921, and the elastic modulus of the tube and fin is 30xc3x97106 psi, since the tube is significantly stiffer than the fin, the thermal stress in the fin will be approximately 90,000 psi. This is generally greater than the yield stress.
The state of the stress is strongly affected by the state of pre-stress between the tube and fin. The corrugated fins are generally brazed to the tube wall. If the thermal expansion coefficient of the tube is slightly less than that of the fin, as would be the case if the tube material was carbon steel and the fin was ferritic stainless steel, then upon cooling from the brazing temperature the fin would contract more than the tube. At room temperature, the fin would be pre-stressed in tension at a level close to the yield stress. This tensile pre-stress would greatly reduce the net compressive stress at operating temperature.
Such has been the case with previous fired vapor generators for absorption heat pumps, where the tube is made of carbon steel and the fins are ferritic stainless steel. However, when the tube is also made of stainless steel, which may be required for resistance to corrosion by the working fluid of the heat pump, the state of pre-stress is either compressive (tube expansion coefficient greater than that of the fin) or the degree of tensile pre-stress is insufficient to overcome the greater amount of thermal expansion in operation (fin thermal expansion coefficient greater than or equal to that of the tube).
The corrugated fin firetube achieves high heat absorption efficiency in a relatively short length by virtue of its small passage size, which provide a small xe2x80x9chydraulic diameterxe2x80x9d conducive to high heat transfer coefficients. However, the same small passage size can be fouled by debris larger than relatively small particle size. A larger passage size, i.e. a wider corrugation pitch, would be less subject to fouling but would significantly reduce the heat transfer coefficient, requiring a longer firetube.
The present invention generally relates to a durable, high-efficiency tubular heater/heat exchanger, such as a firetube heater, generally comprising a tubular member having a fluid inlet end, a fluid outlet end, and a plurality of closely-spaced pin elements bonded to the inside wall of the tube. The present invention may additionally comprise a source for producing heat transfer fluid, including high-temperature gasses, in fluid communication with the fluid inlet end of the tube. During operation, the heat-transfer fluid flows from the fluid inlet end through the tube and through the array of pins. The flow of the fluid is generally parallel to the heat transfer surface, i.e. the tube wall, and perpendicular to the longitudinal axes of the pins. The passageway between adjacent pins is generally large enough to prevent fouling by small particles in the fluid. In addition, since the pin length is usually much greater than its diameter and the temperature gradient of the pins is mostly axial, i.e. from the tip to the base, there is relatively little thermal stress induced in either the pin or the tube, even when the pin is considerably hotter than the tube.
In some embodiments, the source of the heat-transfer fluid may be an internal source, such as a burner secured to the tube at the fluid inlet end. The heat exchanger may also utilize an external source to produce the heat transfer medium.
According to another aspect of the invention, a blocking member is disposed concentrically within the interior core area of the tube defined by the tips of the pins. In one embodiment, the blocking member comprises a core plug which prevents the heat transfer fluid from by-passing the pins, thus increasing thermal effectiveness. The core plug can be tapered to provide a large flow cross-sectional area at the entrance of the pinned array which gradually decreases as the gas flows through the tube. This configuration is useful in, for instance, cooling high-temperature gases to an intermediate temperature before they are forced by the core plug to flow exclusively through the pinned area.
According to another aspect of the invention, the blocking member comprises a series of metal baffles disposed longitudinally along the interior core area of the firetube. The baffles periodically force the heat transfer fluid to flow through the pins. The shape of the metal baffles is not critical; in some embodiments, the baffles block only a portion of the fluid while permitting some of the fluid to flow through. With the baffle array of the present invention, the heat transfer fluid is repeatedly mixed in the areas between the baffles, resulting in a more uniform temperature.
The pins of the present invention generally have a thick cross-sectional area in order to increase conductance and thus prevent overheating. This permits the pins to be made from an inexpensive material of relatively moderate thermal conductivity while obtaining high conductance along the pins. In certain embodiments, the pins comprise carbon steel studs. These studs provide good thermal matching with a conventional carbon steel firetube. In addition, carbon steel pins can be utilized in a stainless steel firetube without incurring excessive thermal stress.
It is a further advantage of carbon steel pins that they can be readily and inexpensively attached to the interior of the tube by means of an arc welding process commonly known as xe2x80x9cstud welding.xe2x80x9d The welding process can be easily automated with programmed positioning, feeding and welding of studs. There is no requirement for a brazing alloy, which would involve time-consuming and labor intensive braze preparation, the use of costly braze alloy, and the potential of corrosion of the braze alloy by the heat transfer medium.
In another aspect of the present invention, a heat exchanger comprises a tube with an interior diameter between 3.5 and 4.25 inches, a fluid inlet end, a fluid outlet end, and a plurality of pins having a diameter between {fraction (5/16)} inch and {fraction (7/16)} inch and a height between ⅝ inch and 1 inch, where the pins are bonded to the interior wall of the tube.
In yet another aspect of the present invention, a vapor generator comprises a tubular member as described above jacketed by a fluid to be heated. The fluid to be heated is admitted into an annulus formed between the exterior of the tube and the interior of a concentric shell surrounding the length of the tube and attached to both ends of the tube. During operation, heat from the high-temperature gas flowing inside the tube is transferred by the pins and the interior of the tube to the exterior of the tube, and ultimately to the fluid contained within the annulus.
According to another aspect, a fluid heater/heat exchanger comprises a tubular heater/heat exchanger as described above, where the tube is disposed concentrically within a larger diameter outer jacket tube. The outer jacket tube is secured to the interior tube at the fluid inlet end and the fluid outlet end, by, for example, a pair of ring flanges. The exterior of the inner tube and the interior of the outer jacket tube define an annulus for containing a fluid to be heated. The outer jacket tube further contains an inlet port for admitting a fluid to be heated to the annulus, and an outlet port for discharging heated fluid from the annulus. The annulus contains at least one baffle element, such that fluid within the annulus is directed by a baffle element to flow in one or more channels.
In one embodiment, the baffle element comprises a helical baffle wound around the exterior of the interior tube to define a helical flow channel between the turns of the helix and the inner and outer tubes.
According to another embodiment, the baffle element comprises a series of longitudinal baffles defining longitudinal flow channels along the length of the interior tube. The direction of flow for each channel can be alternated so that the fluid flows along the length of the interior tube in a first direction, and then flows back in the opposite direction in an adjacent channel. With an even number of longitudinal channels, the input and output ports can be located adjacent to one another at the same end of the outer jacket tube.